Generally, when a file in a user device is infected with a virus file, the virus file will modify the file and add itself into the file. When the file infected with the virus file runs in the user device, the virus file in the file will also be executed. In order to kill the virus, conventional security software usually cleans the virus file in the file.
However, some virus files may corrupt a normal file in the user device seriously. Thus, even if the virus file is cleaned, the file cannot be used any more. This file becomes a defect file.